Business customers have many communications needs that are not satisfied by the ordinary telecommunications services provided by the message network. One of these needs is rapid, convenient, and economical intercommunications among their employees. In some cases, these needs must be fulfilled at a single location and, in other cases, between two or more locations that are distant from one another. This type of service is furnished by a class of equipment called Business Communications Systems (BCS).
The strategy of furnishing business communications service depends on the size of the customer organization, the manner in which it is dispersed over different locations, and the number of people at each location. In addition, the manner in which the needs are fulfilled depends on the service features to be provided in addition to the basic features of intercommunication.
Typically, where customer needs are concentrated at one location, service is provided by a private branch exchange (PBX or PABX) system. This provides local intercommunication services and can also be furnished with a number of other desirable features. The equipment is usually located at the customer premises.
The need to alter PABX switching system design was indicated when direct distance dialing (DDD) was introduced. The extension of DDD service to PABX stations made possible much faster and more efficient calling to points outside the PABX, leading to the introduction of an improved service, called centrex (CTX), to provide direct inward and outward dialing (DID, DOD). The centrex equipment is located at the local telephone company central office building rather than at the customer's location. The centrex system emulates the functions of a standard PABX. In addition to offering the service features required by a large, complex business, centrex gives PABX customers message network service that is comparable to individual line service in speed, flexibility and efficiency. Although the customer no longer has the investment expense of the PABX equipment, there must be a pair of wires extending between the local telephone company office and the customer's location for each extension telephone having a directory number appearance. The centrex system has several advantages including lower cost, better maintenance because of the location of the equipment at the local telephone company office, and also has battery backup which was normally not available with PABX, thereby preserving operability of the telephone system during power failures. Additionally, the digital centrex system such as that provided on a Northern Telecom DMS-100 digital switch could be upgraded automatically simply by replacing the software package with minimum disturbance of the customer's service.
A more recent development involves a special intelligent telephone set known as Electronic Business Set (EBS) used on the Digital Centrex System. The EBS is a multi-line, fully-featured business communication set equipped with a 32-character alphanumeric liquid crystal display that operates over a single pair of wires regardless of the number of assigned directory numbers. These sets contain programmable special function buttons and code generating hardware. At the touch of a single button, special functions can be performed such as hold, interconnecting parties for a conference call, call transfer and the like. Regular telephone sets could be used with the system, but it would then be necessary for the user to key in a sequence of special codes for each function using the twelve-key pad. The Electronic Business Set instruments have the functions of feature keys, signalling and supervision which are accomplished with binary codes using above-voiceband signals amplitude-shift-keyed at 8 kHz.
The above-voiceband signal is a low-level, 8 kHz amplitude-shift-keyed (ASK) 1000 baud message containing 16 bits, using half-duplex transmission. The presence of the 8 kHz carrier indicates a binary "1" and the absence of the 8 kHz carrier is interpreted as a binary "0". Supervision (off-hook/on-hook), feature activation (i.e., hold, conference call) and signaling (dialing) are all activated via 8 kHz messages.
The 8 kHz control signals are transmitted over telephone lines, and because of the attenuation characteristics at 8 kHz of the conventional non-loaded pairs of wires which are primarily intended to carry voice frequency (VF) signals, the subscriber loops are limited to 15,000 ft. using 26 AWG wire. Thus, only those customers within a 15,000 ft. radius of the Centrex III Central Office (DMS-100) can be served. While a system such as this is workable in some customer environments, a customer having multiple locations distributed several kilometers from each other could not employ this system and are therefore denied this desirable service.
A Digital Loop Carrier System currently in use in various telephone environments, known as the Subscriber Loop Carrier System (i.e., SLC-96), is a digital subscriber carrier system which provides for up to 96 subscriber channels between a central office terminal (COT) and a remote terminal (RT) using up to 4 digital lines (Tl) or other digital transmission facilities (e.g., fiber optics).
The shortcoming of previous systems is that the bandwidth is not sufficient to allow direct handling of the 8 kHz control signal.